Targeted hysteresis hyperthermia as a method for treating diseased tissue

ABSTRACT

A method for site specific treatment of diseased tissue in a patient, comprising the steps of: (i) selecting at least a magnetic material which has a magnetic heating efficiency of at least about 4.5×10 -8  J.m./A.g, when magnetic field conditions are equal to or less than about 7.5×10 7  A/s; (ii) delivering the magnetic material to diseased tissue in a patient; and (iii) exposing the magnetic material in the patient to a linear alternating magnetic field with a frequency of greater than about 10 kHz and a field strength such that the product of field strength, frequency and the radius of the exposed region is less than about 7.5×10 7  A/s to generate hysteresis heat in the diseased tissue.

The present invention relates to a method for treating a patient usingtargeted hysteresis therapy. In particular, it relates to a method oftreating patients using site directed hysteresis heat loss.

Diseases of the human body such as malignant tumours are generallytreated by excision, chemotherapy, radiotherapy or a combination ofthese approaches. Each of these is subject to limitations which effectsclinical utility. Excision may not be appropriate where the diseasepresents as a diffuse mass or is in a surgically inoperable locality.Chemotherapeutic agents are generally non-specific, thus resulting inthe death of normal and diseased cells. As with chemotherapy,radiotherapy is also non-specific and results in the death of normaltissues exposed to ionising radiation. Furthermore, some diseases suchas tumours may be relatively resistant to ionizing radiation. This is aparticular problem with the core of a tumour mass.

Hyperthermia has been proposed as a cancer treatment. There is a greatdeal of published evidence to confirm that hyperthermia is effective intreating diseases like cancerous growths. The therapeutic benefit ofhyperthermia therapy is mediated through two principal mechanisms: (1) adirectly tumouricidal effect on tissue by raising temperatures togreater than 42° C. resulting in irreversible damage to cancer cells;and (2) hyperthermia is known to sensitise cancer cells to the effectsof radiation therapy and to certain chemotherapeutic drugs. The lack ofany cumulative toxicity associated with hyperthermia therapy, incontrast to radiotherapy or chemotherapy, is further justification forseeking to develop improved systems for hyperthermia therapy.

Mammalian cells sustain hyperthermic damage in a time/temperature andcell-cycle dependent manner. This cellular response to heat is in turnmodified by a variety of intra- and extra-cellular environmentalfactors. The intra-cellular factors that influence hyperthermic celldamage include intrinsic variation between different species, organs andeven cell lines. The extra-cellular factors include the oxygen andnutritional status of cells, the pH of the extra-cellular mileiu, theabsolute temperature rise and the hyperthermic duration.

Although there is some evidence that neoplastic cells are more sensitivethan their normal tissue counterparts to the effects of hyperthermia,this is not a universal finding and several recent studies havedemonstrated that tissue susceptibility to hyperthermic damage is notstrongly linked to a cell's neoplastic-normal status.

A number of studies have confirmed that hyperthermia and radiotherapyare synergistic. Even small fractions of a degree of temperaturevariation can significantly alter the prospects of cells surviving aradiation insult.

Factors affecting the synergistic action of hyperthermia andradiotherapy include the degree of duration of hyperthermia, thesequence of hyperthermia and radiotherapy, the fractionated and totaldose of radiation, the pH of the extra-cellular milieu, the oxic stateand nutrient status of cells and the histological type and malignantstatus of the cells.

Cells in the central avascular compartment of tumours are invariablyacidotic hypoxic and in a state of nutritional deprivation. All thesefactors appear to potentiate independently the effect of hyperthermia.By the same token, severely hypoxic cells are approximately three timesmore resistant to ionising radiation than oxic cells. Of majorimportance is the fact that although these hypoxic cells might survivethe effects of radiation, hyperthermia can partly overcome thisradioresistance and can potentiate radiotherapeutic killing of acidoticand hypoxic cells.

There are many problems associated with the currently available methodsfor inducing clinical hyperthermia in patients. Normal body tissues andorgans are heat sensitive and at temperatures of greater than 42° C.many tissues will undergo irreversible damage. The current availablemethods of delivering clinical hyperthermia are non-specific and willheat normal tissues as well as tumour cells. Almost all heatingtechniques create heat generation over a broad target area with littlespecificity for diseased tissue, although focussing devices for bothultrasound and electromagnetic heat generation are now being developedto improve the concentration of heat generation in more defined targetareas.

Several techniques are currently available for inducing clinicalhyperthermia either regionally, in selected local regions of specificorgans or over the whole body. Some of these techniques are discussedbelow.

Whole body hyperthermia may be induced by endogenous or exogenous heatsources, but is generally not tolerated above 42° C. withoutanaesthesia. Regional hyperthermic techniques include organ perfusion,various forms of electromagnetic radiation, or ultrasound.

Plain wave electromagnetic or ultrasound heating is limited by poortissue penetration and a rapid decline of energy with increasing depth.

Ultrasound at frequencies of from 0.3 to 3 MHz is limited by theperturbations induced by tissue interfaces such as air, bone etc.However, improved focussing devices are being developed that may makethis a more acceptable form of heating for deep tissues.

Microwave heating at frequencies between 434 and 2450 MHz has been used,although there is generally poor tissue penetration. Phase array devicesare able to focus microwave energy in deep tissues, but variation in theheating effect remains a problem.

Radiofrequency waves at frequencies up to 434 MHz have been used withsome success. These heating techniques include both dielectric andinductive modalities and can result in relatively even tissue heating.However, focussing for deep organ heating using inductive currentremains a problem.

There are two basic requirements for such therapies to be effective.First, there is a need to localise the treatment to the target site.Second, there is a need to maximise heating within the diseased tissuewhile maintaining hyperthermia therapy within safe operating limits forthe patient.

While considerable success has been observed in treating superficialtumours using hyperthermia therapy, there remains a need for a method ofselectively targeting and treating diseased tissue in a patient. Majorlimitations due to insufficient penetration depth and poor focussingcapabilities of externally applied microwave or ultrasound beams havegrossly restricted a physicians ability to deliver an adequate heat loadto deep seeded diseased without any unacceptable level of coincidentdamage to surrounding healthy tissue. The present invention seeks toameliorate at least the problems associated with penetration depth andinadequate localisation of heat when using hyperthermia therapy.

SUMMARY OF THE INVENTION

The present invention provides an improved method for site specifictreatment of diseased tissue in a patient, which comprises the steps of:

(i) selecting at least a magnetic material which has a magnetic heatingefficiency of at least about 4.5×10⁻⁸ J.m./A.g, when magnetic fieldconditions are equal to or less than about 7.5×10⁷ A/s;

(ii) delivering the magnetic material to diseased tissue in a patient;and

(iii) exposing the magnetic material in the patient to a linearalternating magnetic field with a frequency of greater than about 10 kHzand a field strength such that the product of field strength, frequencyand the radius of the exposed region is less than about 7.5×10⁷ A/s togenerate hysteresis heat in the diseased tissue.

Preferably, steps (i) to (iii) in the method are repeated until thediseased tissue has been destroyed or treated sufficiently to amelioratethe disease.

The magnetic material employed in the method of the invention must havea magnetic heating efficiency (MHE) of greater than about 4.5×10⁻⁸J.m./A.g, when magnetic field conditions are equal to or less than about7.5×10⁷ A/s. Preferably, a magnetic material is selected which has a MHEof greater than about 7×10⁻⁸ J.m./A.g, when magnetic field conditionsare equal to or less than about 7.5×10⁷ A/s. Most preferably, a magneticmaterial is selected which has a MHE of greater than about 1×10⁻⁷J.m./A.g, when magnetic field conditions are equal to or less than about7.5×10⁷ A/s.

Advantages gained by using a magnetic material with a large MHE include:

1) improved therapeutic effectiveness by virtue of the fact that highertumour temperatures can be reached more quickly (the effectiveness ofhyperthermia therapy improves markedly as temperature is increasedbeyond 42° C.);

2) reduced toxic side effects because:

i. less microcapsules need to be used to achieve therapeutic heating intumours (advantageous if the microcapsules have any intrinsic toxicity),

ii. a lower magnetic field strength, H, can be used,

iii. more rapid heating of the tumour may be achieved which implicatesless of the healthy tumour tissue immediately surrounding the tumour(the longer time required to heat the tumour the more the immediatelysurrounding tissue will be heated by thermal conduction);

3) increased likelihood of successful treatment especially for tumoursthat would otherwise be expected to only receive a marginal benefit;

4) the techniques have a wider applicability for the treatment ofdifferent types of cancer;

5) using reduced field strengths eases engineering difficultiesassociated with machine design;

6) using reduced field strengths means reduced electrical powerconsumption and cooling requirements while running the machine.

The selection of magnetic material suitable for use in the presentinvention is based on the MHE of the material. MHE may be calculatedusing the following formula: ##EQU1## where P_(hyst) is the heatingpower generated by magnetic hysteresis loss effects (units W/g), H isthe amplitude of the applied magnetic field (units A/m) and f is thefrequency of the applied magnetic field. The major limitations to thegeneration of heat by magnetic hysteresis for the purposes of treatingdiseased tissue, arise from the effect a time varying magnetic field hason living tissue. In general these effects increase as the product of fand H increases. Hence, it is essential that P_(hyst) be maximisedsubject to minimising the product of f and H.

Further, P_(hyst) can be calculated using the following formula:

    P.sub.hyst =f.W (W/g)                                      (2)

where W is the hysteresis heat energy (units J/g) generated in themagnetic material during each cycle of the applied magnetic field and fis the frequency as before.

Combining equations (1) and (2) and eliminating f it can be seen thatMHE can be calculated once H and W are known. W must be measuredexperimentally for each value of H. This may be achieved in the mannerdescribed herein. The MHE is then calculated from equations (1) and (2).

W can be determined using several different methods described below:

1) For magnetic hysteresis measurements a Vibrating Sample Magnetometer(VSM) is used to measure W. A known quantity (typically less than 1 g)of the magnetic powder is fixed in a non-magnetic, non-metallic VSMsample container using a non-magnetic epoxy. Samples are in an initiallydemagnetised state and the value of W is determined at successivelyhigher field strengths.

2) A 50 Hz Alternating Field Magnetometer is also be used to measure W.Samples prepared as for the VSM are placed inside a small coil. Thissmall coil is then placed between the pole pieces of a magnet thatproduces a magnetic field alternating at 50 Hz. The voltage induced inthis coil is equal to N.dB/dt where N is the number of turns in thecoil. This voltage signal is integrated, corrected for air flux and aplot of magnetisation, M, vs H is generated. This is the hysteresis loopthe area of which equals W.

3) An alternative to these methods is to take a known quantity of themagnetic powder, typically 125 mg, and disperse it in 5 ml of agar gel(3% agar dissolved in warm water. The agar solidifies when cooled backto room temperature.) A temperature probe is inserted into the gel andthe whole exposed to the alternating magnetic field of desired strength.From the resultant curve of Temperature vs Time it is possible tocalculate W at H.

Any magnetic material which exhibits hysteresis and which has a MHE ofgreater than 4.5×10⁻⁸ J.m./A.g, when magnetic field conditions are equalto or less than about 7.5×10⁷ A/s may be used in the present invention.Preferably, the magnetic materials are ferromagnetic materials.Ferromagnetic materials may include elements such as iron, manganese,arsenic, antimony and bismith, but are not limited to such elements.Classes of materials from which the magnetic material may be selectedinclude CrO₂, gamma-ferric oxide (both cobalt treated and non-treated)and metallic iron, cobalt or nickel. Also ferrites of general formMO.Fe₂ O₃ where M is a bivalent metal, e.g. Mg, Mn, Fe, Co, Ni, Cu, Zn,Cd or Li, cobalt treated ferrites or magnetoplumbite type oxides (Mtype) with general form MO.6Fe₂ O₃ where M is a large divalent ion suchas Ba, Sr or Pb are all potentially useful magnetic materials in thisapplication. Further, superparamagnetic, single domain particles may beused as the magnetic material. Most preferably, the ferromagneticmaterial is selected from the class of ferromagnetic materials known asgamma-ferric oxide, (γFe₂ O₃).

Examples of suitable ferromagnetic materials from which the magneticmaterials might be selected include Co treated gamma-ferric oxide, somenon cobalt treated gamma-ferric oxides, cobalt treated ferrites andchromium dioxide.

The method of the invention provides a means to increase temperature inthe area of diseased tissue to above 41° C. to decrease the viability ofmalignant cells. A decrease in the viability of malignant cells resultsin either cell death or increased cell sensitivity to the effects ofionizing radiation or chemotherapeutic drugs.

The amount of hysteresis heat that is generated in a magnetic materialduring each cycle of an applied magnetic field is given by W. To turnhysteresis heat energy into power that is capable of heating tissue, themagnetic field must have a high frequency of alternation. Duringtreatment, patients are placed into a machine that generates a magneticfield with strength H and frequency f. The higher the frequency thegreater the rate of heating in the tissues that contain the magneticmicrocapsules. However, the physiological response to high amplitude,high frequency magnetic fields limits the field amplitude and frequencythat can be used in any clinical application. These limitations resultfrom nerve muscle activation and eddy current heating which depends,inter alia, on the electrical conductivity of the tissue. Both of theseare as a result of the electric fields induced in the tissue by themagnetic field. The size of these potentially deleterious inducedelectric fields is proportional to the square of the product of H, f andthe radius of the exposed area, r, normal to the direction of the field.The product of H, f and r largely defines the magnetic field conditions.The product of H, f and r should not exceed a value of about 7.5×10⁷A/s., ie H.f.r≦7.5×10⁷ A/s. To illustrate this point consider the caseof whole body exposure to a magnetic field applied along the body axis.In this case r is typically 0.15 m so the product of f and H should notexcede about 5×10⁸ A/m.s.

The magnetic material used in the invention may be delivered to thediseased tissue in a patient by any means known in the art. Suitableroutes of administration might include: intratumoral, peritumoral andintravascular administrations (eg intra-arterial, intraperitoneal,subcutaneous or intrathecal injections). Preferably, the magneticmaterials are delivered to the diseased tissue via the arterial orvenous blood supply.

Preferably, the magnetic material is mixed in a liquid emulsion or isbound into microcapsules which may then be mixed with a suitablebiocompatible medium for delivery into a patient. Most preferably themagnetic material is bound in a matrix material to form a microcapsule.Most magnetic particles themselves are, typically, too small and toodense to enable optimum delivery to the site of diseased tissue.Therefore, they are desirably encapsulated in microcapsules. Importantproperties of microcapsules are their density and their diameter. Thedensity effects the efficiency of their carriage by the blood stream tothe site of immobilisation in the diseased tissues vascular networkwhile the size determines the proximity of the point of immobilisationto the diseased tissue.

Preferably, the magnetic material is bound in a matrix material whichdoes not adversely effect the hysteresis or eddy current heatingproperties of the magnetic particles. The non-toxic binder or matrixmaterial may comprise any of the suitable non-toxic materials which arewell known in the microencapsulation art. Suitable materials include,for example, proteins, polymeric resins such as styrene-divinylbenzene,biopol, albumin, chitoxan etc.

In a preferred form of the invention, the microcapsules are adapted tobind or absorb or contain a cytotoxic material which is released uponheating of the microcapsule. For example the microcapsule may becomposed of a porous, heat sensitive material which is non-toxic to and,preferably, inert to or compatible with animal tissue and which hasembedded therewithin suitable magnetic material. The pores in thematerial are desirably filled with the cytotoxic compound. Uponhysteresis heating the micro-particles are capable of expanding, therebypermitting the release of the cytotoxic compound. Such particles should,however, be resistant to melting upon hysteresis heating. Thus, the useof such particles in the method of the present invention provides asingle device with which combined chemotherapy and thermotherapy can beachieved to treat diseased tissue in a patient.

According to a further embodiment of the invention, an ionizingradiation source may be applied to the locus of the diseased tissue inconjunction with a magnetic field, said tissue having microcapsules asherein described included therein. The radiation source may bemicrocapsules which contain a radioactive compound such as Yttrium-90 ordelivered from an external radiation source.

DETAILED DESCRIPTION OF THE INVENTION IN THE DRAWINGS

FIG. 1 shows a representative hysteresis loop illustrating howmagnetisation of a magnetic sample (y-axis) varies as the applied field(x-axis) is cycled. A hysteresis loop is produced over one completecycle. The area of this loop gives a W value.

FIG. 2 shows the MHE as a function of applied magnetic field strengthfor a selection of magnetic materials.

FIG. 3 shows the heating of selected magnetic materials when exposed tohigh frequency magnetic fields.

FIG. 4 illustrates the heating effectiveness of microcapsules when usedto heat well perfused living tissue using an amount of microcapsulesthat equates to a clinically relevant dose.

FIG. 5 illustrates the heating effectiveness of microcapsules when usedto heat liver tumours to therapeutic temperatures whilst leaving thesurrounding healthy tissue unaffected.

FIG. 6 illustrates the therapeutic effectiveness of the subject methodin destroying diseased tissue.

Generation of heat in a range of 41° C. or higher (hyperthermia) causesirreversible damage to diseased cells. Thus, diseased tissue may betreated by elevating the diseased tissue's temperature (thermotherapy)as a result of hysteresis heat loss from suitable magnetic materials.Preferably, the heat generated by hysteresis heating is in the range of42° C. to about 60° C.

The present invention may be used to treat any diseased tissue which maybe sensitive to thermotherapy, chemotherapy or radiotherapy or acombination of thermotherapy and chemotherapy or radiotherapy.Preferably the invention is employed to treat cancerous growths ortissue which contains one or more tumours. Although the following willdiscuss the use of microcapsules and the method in terms ofcancer/tumour treatments, it should be appreciated that applications ofthe method extend beyond the cancer/tumour treatments and the use onlyof microcapsules.

When a magnetic substance is subjected to a magnetic field with astrength that varies cyclically, some heat is generated due to magnetichysteresis loss. The amount of heat generated per cycle depends on thehysteresis loss which varies for each different ferromagnetic materialand for different magnetic field conditions. For multi-domainferromagnetic materials the hysteresis loss is independent of particlesize. Magnetic particles embedded around a tumour site and placed withinan oscillating magnetic field will heat up to a temperature dependent onthe magnetic properties of the material, the strength of the magneticfield, the frequency of oscillation and the cooling capacity of theblood flow in the tumour site.

Energy in the form of heat is produced as a result of magnetichysteresis loss in a ferromagnetic sample whenever it is subjected to analternating magnetic field. The amount of hysteresis energy that isgenerated by the magnetic material during each cycle of a magnetic fieldis given by W (J/g). The heating power generated by hysteresis effectsis thence given by

    P.sub.hyst =f.W=f/p§HdB W/g                           (3)

where f is the frequency of alternation of the applied magnetic fieldand p is the density of the magnetic material. The quantity §HdB isequivalent to the area of the hysteresis loop that characterises themagnetic sample under a particular set of magnetic field conditions (atypical example of a hysteresis loop is shown in FIG. (1)).

To determine the minimum amount of heat that must be generated from themagnetic microcapsules for them to be an effective therapeutic agent,P_(tumour) (W/cm³) needs to be ascertained. P_(tumour) is given by:

    P.sub.tumour =f.W.sub.c.p.v.n (W/cm.sup.3)                 (4).

where f the frequency of the applied magnetic field in Hz,

W_(c) the amount of heat energy generated by hysteresis effects in theinjected magnetic microcapsules each cycle of the magnetic field, unitsof J/g,

p the density of the injected microcapsules in units of g/cm³,

v the volume of each microcapsules in units of cm³,

n the number of microcapsules per cm³ of tumour tissue.

P_(tumour) in essence represents the rate of tumour tissue heating. Whenrelated to the present invention P_(tumour) should be large enough thatit causes the temperature of the tumour tissue to increase from bodytemperature to a temperature that is lethal to the diseased cells over areasonable period of time. Moreover, P_(tumour) should be great enoughto overcome tissue cooling influences such as blood flow and tissuethermal conductivity. Preferably, P_(tumour) is greater than 60 mW/cm³.Most preferably it is greater than 80 mW/cm³ and desirably it is greaterthan 100 mW/cm³.

To obtain P_(tumour) values in the preferred range, suitable values needto be selected for the variables f, W, p, v and n.

Available experimental data concerning human responses to oscillatingmagnetic fields is limited. Such data has lead to the identification ofan optimal operational frequency range between about 10 kHz and 100 MHz.For frequencies less than this there is the danger of involuntaryneuromuscular activation and for higher frequencies limitations begin toarise due to reduced penetration of the electromagnetic energy into thetissue. Therefore, the frequency should be maintained within the range10 kHz to 100 MHz. Preferably the frequency is maintained with the range10 kHz to 500 kHz. Most preferably the frequency is maintained with therange 10 kHz to 100 kHz and desirably it would not exceed 50 kHz. Forexample, the frequency might be 20 kHz.

W (J/g) is an intrinsic property of the magnetic material incorporatedinto the microcapsules. W increases as the maximum amplitude of theapplied magnetic field H, is increased. There are, however, limits tothe amount that H can be increased when applying the method to patients.Such limits are also dependent on the frequency used and the area oftissue exposed to the magnetic field. The magnetic material chosen foruse in the microcapsules should have a MHE of at least about 4.5×10⁻⁸J.m./A.g, when magnetic field conditions are maintained within safeoperating limits for a patient. Preferably, a magnetic material isselected which has a MHE of greater than about 7×10⁻⁸ J.m./A.g, whenmagnetic field conditions are equal to or less than about 7.5×10⁷ A/s.Most preferably, a magnetic material is selected which has a MHE ofgreater than about 1×10⁻⁷ J.m./A.g, when magnetic field conditions areequal to or less than about 7.5×10⁷ A/s.

The requirements for magnetic field strength and frequency will also,depend on the properties of the microcapsules. These properties areaccounted for by the parameters: p (g/cm³), v (cm³), and n per cm³.

Microcapsules used in the method of the invention should be of asuitable size to pass through a patient's vasculature network and becomedispersed and embolised within diseased tissue (with or without theassistance of vasoactive agents). The capsules used, should be capableof becoming entrapped in the precapillary and capillary network oforgans, tumours or tissues without passing back into the general venouscirculation. Preferably, the microcapsules should be larger than about10 microns in diameter so that they lodge in the tumour vascular supply,and they should be smaller than about 500 micron so that they don'tembolise in the blood vessels before reaching the tumour. Mostpreferably the microcapsules range in size between about 10 to 100microns, with 30 to 40 microns being most desirable.

Smaller microcapsules less than 10 microns may also be used in themethod if they become incorporated into tumour tissues by the process ofendocytosis.

Moreover, the density of ferromagnetic material should be such, so as toallow the microcapsules to be carried by the bloodstream in a patient.The microcapsules preferably possess a density in the range 1 to 5g/cm³. Most preferably, the density should be between 1.8 to 3 g/cm³.Desirably, the density is in the range 1.8 to 2.2 g/cm³, for example 2g/cm³.

A number of different methods may be used to prepare the microcapsulesusing a diverse range of matrix materials and manufacturing techniques.In one preferred form of this invention, the microcapsules containcobalt treated γFe₂ O₃ particles as the ferromagnetic material, boundtogether using a Biopol matrix (a copolymer of (R)-3-hydroxybutyric acidand (R)-3-hydroxyvaleric acid). Using this matrix, magneticmicrocapsules in a density range of 1.8-2.2 g/cm³ and in a size range20-50 microns can be obtained.

The microcapsules may be formed of bio-degradable or non-biodegradablematerial. Preferably, the microcapsules used in the present inventionare not degradable and are permanently lodged in the tumour vascularnetwork. Thus, they can be used repeatedly to generate localised tumourheating. By subjecting the tumour bearing organ to a magnetic field, theferromagnetic material contained within the microcapsules will heatcausing highly localised tumour heating, with preservation of thesurrounding normal parenchyma.

Microcapsules may be formed by any suitable known technique (See forexamples, the "Encyclopedia of Chemical Technology" KlRKO-OTTHER, Vol.15 Wiley-Interscience). For example, ferromagnetic particles may beadded to a protein solution, such as an albumin solution. The resultingprotein solution should preferably then be added to an oil phase whichis continually agitated thereby forming an emulsion. The proteinaceousmaterial may then be cross-linked using heat, or chemical reagents suchas glutaraldehyde to form microcapsules having ferromagnetic particlestrapped therein.

In an alternative method, ferromagnetic particles may be added to asolution containing Biopol in dichloromethane. The mixture is preferablythen dropped into a beaker containing poly-vinyl alcohol or the likewhile being mixed with a homogenising mixer. The mixture should then beleft to slowly mix for a suitable period of time to allow thedichloromethane to evaporate. Microcapsules thus formed, may then bewashed and size fractionated.

Following preparation of the microcapsules, the preparation may be sizefractionated to select particles of a preferred size for use in themethod of the invention. Preferably the preparation is also densityfractionated to select for particles of a preferred density.

In one embodiment of the invention, microcapsules composed of a materialwhich is non-toxic to and preferably inert to or compatible with ananimal and which has incorporated there within at least a ferromagneticmaterial, are targeted (either directly or indirectly) to and deliveredto diseased tissue in a patient. The microcapsules should then besubjected to a magnetic field of less than 7.5×10⁷ A/s for sufficienttime to treat the diseased tissue. The time required to treat suchtissue will depend on the heat generated in the microcapsules whichdepends on magnetic field used and the properties of the microparticlesemployed.

A variety of administration routes are available for use in thetreatment of a human or animal patient. The particular mode ofadministration selected will depend, of course, upon the particularcondition being treated and the number of microcapsules required fortherapeutic efficacy. The method of this invention, generally speaking,may be practised using any mode of administration that is medicallyacceptable, which is capable of selectively delivering microcapsules todiseased tissue without causing clinically adverse effects and which iscapable of delivering microcapsules to diseased tissue in a patient,such that the microcapsules are distributed in a substantially evenmanner throughout the diseased tissue. Such modes of administrationmight include parenteral (eg. subcutaneous, intramuscular intraarterialand intravenous) routes.

In one embodiment of the invention microcapsules are preferablydelivered by injection of a microcapsule suspension into the arterial(or portal venous) blood supply of the diseased tissue. Compositionssuitable for parenteral administration conveniently comprise a sterileaqueous preparation of the capsules which is preferably isotonic withthe blood of the recipient. The sterile preparation may be an injectablesolution or suspension in a non-toxic parentally-acceptable diluent orsolvent. Among the acceptable vehicles and solvents that may be employedare water, ringer's solution and isotonic sodium chloride solution.

The number of microcapsules per unit volume of tissue that may be usedin the method will depend entirely on the amount of diseased tissue thatis to be treated in a patient. Preferably, the number of microcapsuleper gram of tissue is in the range of 5,000 to 300,000(microcapsules/g). Most preferably, the range if 10,000 to 100,000, with40,000 to 70,000 being desirable. For example, n is about 60,000microcapsules per cubic centimeter of tumour tissue.

If, for example, the invention is used to treat a tumour or canceroustissue, the microcapsules should be embolised into the vascular networkof the tumour containing tissue so that the capsules concentrate withinthe tumour compartment while sparing the surrounding normal parenchyma.

The vasculature of the border area between normal tissue and theinfiltrating tumour consists mainly of arterioles with adrenergicreceptors, whereas vessels within the tumour lose these characteristics.Although the tumour vascular bed has little blood flow regulation, thearteriolar supply to the tumour which resides in the adjacent normaltissue is subject to normal vasomotor control. This loss of blood flowregulation in tumours underscores the principal reason why tumourscannot dissipate heat at the same rate as the ambient normal tissue whensubjected to conditions of increased heat input, thus resulting inpreferential heating of tumour tissue.

Progressive tumour growth results in the central region of tumoursbecoming relatively avascular and hypoxic. These areas do usually stillcontain collapsed blood vessels capable of transmitting blood flow underthe influence of some vasocactive agents. The ability to lodgemicroparticles containing ferromagnetic material into the vascular bedof tumour tissue can be enhanced by manipulation of the blood flow ofthe tumour and surrounding tissues using vasoactive agents. In oneembodiment of the invention the microcapsules are preferablyadministered to diseased tissue under the control of vasoactive drugs.Most preferably, normal parenchyma is treated with vasoconstrictivedrugs to prevent microcapsules from entering that tissue.

The combined delivery of microcapsules loaded with ferromagneticmaterials, and vasoactive agents such as Angiotensin II, Noradrenalineplus beta blockade, Vasopressin, Epinephrine or other vasoactive agentsmay open up the collapsed microcirculation in the central portions oftumours and provide access for the deliver of microcapsules into theseregions. On cessation of the effect of the vasoactive agent the centralportions of tumours would return to hypovascular and hypoxic state, butwould be rendered susceptible to hyperthermia damage.

The phenomenon of physiologic unresponsiveness of tumour blood vesselsmay thus be manipulated to allow microcapsules to be selectivelytargeted to tumour tissue. The infusion of vasoconstrictor drugs intothe arterial circulation of tumour bearing organs will cause a transientvasoconstriction of the vessels supplying the normal tissue but notthose supplying the tumour. When microcapsules are introduced into thearterial circulation immediately following infusion of vasoactive drugs,the microcapsules will be preferentially directed to and trapped in thetumour vascular network and not normal tissues. The effect of thevasoactive drug will wear off within several minutes. However, by thenthe microcapsules will be firmly lodged in the tumour capillary network.Conversely, vasodilatory drugs may be used to selectively targetradioprotectant or thermoprotectant agents to the normal non-tumourtissues.

The advantages of delivering ferromagnetic microcapsules via thevascular route compared to direct injection may be summarised as:

(i) arterial delivery of microcapsules in combination with vasoactivedrug treatment allows even or substantially even distribution of themicrocapsules through diseased tissue without delivery of microcapsulesto normal parenchyma. In contrast, injection of microcapsules directlyinto diseased tissue does not result in even or substantially evenmicrocapsule distribution. In such circumstances, microcapsules wheninjected into diseased tissue, focus at highest concentration, aroundthe injection site. The density of microcapsules per unit volume ofdiseased tissue progressively decreases when moving away from the focalpoint of injection.

(ii) arterial delivery of microcapsules reduces the risk that secondarytumours will be missed, as might be the case with microcapsule deliveryvia injection.

(iii) arterial delivery of microcapsules avoids the need for surgicalaccess to all tumours.

(iv) arterial delivery of microcapsules avoids the likelihood of tumourcells being spread, which might occur when a tumour is punctured by aneedle.

According to a further embodiment of the invention, microcapsules loadedwith ferromagnetic particles are introduced into a tumour, or a tissuecontaining a tumour, in conjunction with one or more vasoactive agents.A magnetic field is then applied to the locus of the tumour to induceheating either by hysteresis heating or eddy current heating of theferromagnetic particles.

Any magnet capable of delivering desired field strengths and frequenciesmay be employed in the present invention. Suitable magnets include aircored coils or laminated silicon iron cored electromagnets or ferritecored magnets. Magnets may be portable.

A number of different devices may be used to generate the appropriatelyconditioned time varying magnetic field.

An alternatingmagnetic field is described mathematically by:

    H(t)=H.Sin (2πft)                                       (5)

where H(t) is the applied field strength at time t, H is the maximumamplitude of the applied field and f is its frequency of alternation.Any device which is capable of generating such a field may be used inthe present invention.

When an alternating field is employed, the device used to generate thefield, preferably uses an inductive element connected to a capacitorbank thereby forming either a series or parallel resonant circuit. Theresonant circuit is preferably driven by a suitable power supply with amatching transformer. A magneto-motive force is preferably produced by asuitable inductive element such as a coil or a pair of coils. In somecases the coils may be wound onto a non-conducting high permeabilitycore for improved operation. In one example, the coils may be fabricatedfrom a low resistivity metal such as copper. The coils are preferablycooled by a suitable cooling means which might include, for example,circulating water or liquid nitrogen. Furthermore, the coils may befabricated from hollow tube through which the coolant flows or they maybe composed of many small diameter strands of wire, e.g. Litz wire,cooled by immersion in the coolant.

Preferably the device used in the method is capable of producing therequired magnetic field conditions in a region of space large enough toaccommodate a human patient. Moreover, the device is preferably capableof maximizing the MHE of the microcapsules.

Further features of the present invention are more fully described inthe following Examples. It is to be understood, however, that thisdetailed description is included solely for the purposes of exemplifyingthe invention, and should not be understood in any way as a restrictionon the broad description as set out above.

EXAMPLE 1 Selection Of Ferromagnetic Material

This example compares the heating efficiency of a large number ofdifferent ferromagnetic materials subjected to an alternating magneticfield.

The following ferromagnetic materials (see table 1) were obtained fromindustry sources:

    ______________________________________                                        Ferromagnetic Material                                                                            Industry Source                                           ______________________________________                                        Co-γFe.sub.2 O.sub.3 (S11)                                                                  Bayer Chemicals                                           γFe.sub.2 O.sub.3 (8115)                                                                    Bayer Chemicals                                           Co treated ferrite (PK5134M)                                                                      Bayer Chemicals                                           Chromium Dioxide    Dupont                                                    Alnico              David Oriel Industries                                    Hexaferrite         David Oriel Industries                                    Magnequench Powder A                                                                              Delco Remy (GM)                                           Magnequench Powder B                                                                              Delco Remy (GM)                                           ______________________________________                                    

The heating efficiency of these magnetic materials was examinedaccording to the methods described earlier. The analysis involvedmeasuring the hysteresis loop of each material in either a VibratingSample Magnetometer or a 50 Hz Alternating Field Magnetometer. Theresults of these analysis are presented graphically in FIG. 2.

A direct measurement of the heat output from small samples of each ofthe above magnetic materials when exposed to high frequency magneticfield (53 kHz, 28 kA/m) was also made. The results of this measurementare shown graphically in FIG. 3. These results show the clearly superiorheating characteristics of the γFe₂ O₃ materials at this particularfield strength (both Co treated and untreated) reflecting their higherMHE factors.

Preparation Of Ferromagnetic Microcapsules

γFe₂ O₃ particles having a maximum MHE of 1.05×10⁻⁷ J.m./A.g, when thefield strength was 47.1 kA/m were obtained from Bayer Chemicals. 1 g ofγFe₂ O₃ particles was thoroughly mixed with a 6 ml solution containing15% Biopol (Fluka Chemie, Switzerland) in dichloromethane. This mixturewas then dropped into a beaker containing 150 ml of 0.25% poly-vinylalcohol (2.5 g of PVA 87-89% hydrolyzed, MW 124,000-186,000 dissolved in1 Liter of water) while being mixed with a homogenising mixer set at3900-4000 rpm. The mixture was then left mixing for 10 minutes afterwhich it was left to mix very slowly for 60 minutes to allow all thedichloromethane to evaporate.

Microcapsules thus formed were washed successively through 63, 45 and 20micron sieves. The fraction between 20 and 45 microns was kept. Thecapsules were then floated on diiodomethane, slightly diluted withacetone to give a specific gravity of 2.2. Any microcapsules that sinkwere discarded. The remainder were then washed and floated ondiiodomethane diluted to a specific gravity of 1.8. The microcapsulesthat sink were reclaimed and washed ready for use.

Heating Of Living Tissue

This experiment shows that microcapsules when prepared by the abovemethod can be used to heat well perfused living tissue using an amountof microcapsules that equates to a clinically relevant dose.

Magnetic microcapsules produced according to the above method wereadministered to rabbit kidneys by infusion through the renal artery.Three different quantities of microcapsules were used in separatetrials, 50 mg, 25 mg and 12.5 mg (corresponding to concentrations ofapproximately 125,000, 62,500 and 31,250 microcapsules per cubiccentimeter of tissue). Thermometer probes (fluoroptic probes, LuxtronCorp). were fixed in place to measure temperatures throughout thekidney. The animals were then placed into apparatus that can produce analternating magnetic field with strength 28 kA/m and frequency 53 kHzwith radius of 0.05 m (MHE=6.1×10⁻⁸ : Magnetic field conditionsf.H.r=7.4×10⁷ A/s.). The field was then switched on and temperaturesmonitored for approximately 15 minutes. Typical data are shown in FIG.4. Given that the kidney is the most highly perfused organ in the body,and hence should be the most difficult to heat, this data providesevidence that the method described is capable of heating tissue in eventhe most extreme conditions.

Targeted Tumour Heating

This experiment demonstrates how the above method can be used to heatliver tumours to therapeutic temperatures whilst leaving the surroundinghealthy tissue unaffected.

Small segments of VX2 carcinoma were implanted just under the liversurface of half lop rabbits. Once the tumours had grown to a size ofapproximately 1 cm³, 50 mg of microcapsules, produced according to theabove method, were infused using approximately 5 ml saline through a 0.8mm O.D. catheter inserted into the cystic artery adjoining the hepaticartery feeding into the liver of the rabbits. Thermometer probes werefixed in place to measure temperatures in the necrotic core of thetumour, the growing edge of the tumour, in nearby normal liver tissueand in more distant part of the liver. The rabbits were then placed inthe magnetic field apparatus as described above (f.H.r=7.4×10⁷), thefield switched on and temperatures monitored. After a period ofapproximately 1 hour the field was switched off and temperatures wereallowed to return to normal. FIG. 5 shows data from one of theseprocedures. The differential heating between tumour and normal tissue isclear. The tumour was heated to the therapeutic threshold temperature of42° C. and maintained at that temperature while the normal livertemperature did not exceed 40° C.

Evaluation Of Therapeutic Effectiveness

This experiment examined the therapeutic effectiveness of the abovemethod.

The procedure described above was repeated under sterile conditions.Tumour temperatures were maintained at or above 42° C. for a period of30 minutes. The rabbits were then revived and kept for a period ofeither 7 days or 14 days. At these time points rabbits were sacrificed,their livers excised and tumour weights recorded. These weights arepresented in FIG. 6 along with weights for a control group that receivedno treatment. The results show a dramatic and clear difference in tumourweights at 14 days post treatment.

The claims defining the invention are as follows:
 1. A method for sitespecific treatment of diseased tissue in a patient, comprising the stepsof:(i) delivering a magnetic material which has a magnetic heatingefficiency of at least about 4.5×10⁻⁸ J.m./A.g. when magnetic fieldconditions are equal to or less than about 7.5×10⁷ A/s to diseasedtissue in a patient; and (ii) exposing the magnetic material in thepatient to a linear alternating magnetic field with a frequency ofgreater than about 10 kHz and a field strength such that the product offield strength, frequency, and the radius of the exposed region is lessthan about 7.5×10⁷ A/s to generate hysteresis heat in the diseasedtissue.
 2. The method of claim 1, wherein steps (i) to (ii) are repeateduntil the diseased tissue has at least been treated sufficiently toameliorate the disease.
 3. The method of claim 1, wherein the diseasedtissue contains at least a cancerous growth or contains one or moretumours.
 4. The method of claim 1, wherein the magnetic material has amagnetic heating efficiency of greater than about 7×10⁻⁸ J.m./A.g., whenmagnetic field conditions are equal to or less than about 7.5×10⁷ A/s.5. The method of claim 1, wherein the magnetic material has a magneticheating efficiency of greater than about 1×10⁻⁷ J.m./A.g., when magneticfield conditions are equal to or less than about 7.5×10⁷ A/s.
 6. Themethod of claim 1, wherein the magnetic material comprises aferromagnetic material which contains at least an element selected fromthe group consisting of iron, manganese, arsenic, antimony, and bismith.7. The method of claim 1, wherein the magnetic material is selected fromcompounds within the group of CrO₂, metallic iron, cobalt, nickel,gamma-ferric oxide, cobalt-treated gamma-ferric oxide, ferrites of thegeneral formula MO.Fe₂ O3 where M is a bivalent metal, cobalt-treatedferrites, or magnetoplumbite-type oxides (M type) with the generalformula MO.6Fe₂ O₃ where M is a large divalent ion.
 8. The method ofclaim 7, wherein the magnetic material is a compound within the group ofcobalt-treated gamma-ferric oxide compounds.
 9. The method of claim 7,wherein the magnetic material is a compound within the group ofun-modified gamma-ferric oxide compounds.
 10. The method of claim 7,wherein the magnetic material is a compound within the group of chromiumdioxide compounds.
 11. The method of claim 1, wherein the magneticmaterial is mixed in a biocompatible liquid emulsion prior to deliveryinto a patient.
 12. The method of claim 1, wherein the magnetic materialis bound in a matrix to form microcapsules.
 13. The method of claim 12,wherein the microcapsules range in size from about 10 to about 100microns.
 14. The method of claim 13, wherein the microcapsules range insize from about 20 to about 50 microns.
 15. The method of claim 14,wherein the microcapsules range in size from about 30 to about 40microns.
 16. The method of claim 12, wherein the microcapsules areadapted to bind, absorb, or contain a cytotoxic material which isreleased upon heating of the microcapsule.
 17. The method of claim 12,wherein an ionizing radiation is applied to the diseased tissue inconjunction with the magnetic field.
 18. The method of claim 17, whereinthe radiation is applied by microcapsules which contain a radioactivecompound.
 19. The method of claim 1, wherein said exposing produces arate of tissue heating greater than 60 mW/cm³.
 20. The method of claim19, wherein the rate of tissue heating is greater than 80 mW/cm³. 21.The method of claim 19, wherein the rate of tissue heating is greaterthan 100 mW/cm³.
 22. The method of claim 1, wherein the linearalternating magnetic field has an operational frequency of from about 10kHz to 100 MHz.
 23. The method of claim 1, wherein the linearalternating magnetic field has an operational frequency of from about 10kHz to 500 kHz.
 24. The method of claim 1, wherein the linearalternating magnetic field has an operational frequency of from about 10kHz to 100 kHz.
 25. The method of claim 1, wherein the magnetic fieldhas an operational frequency of 20 kHz.
 26. The method of claim 12,wherein the microspheres are of a suitable size to pass through apatient's vasculature network and become dispersed and embolized withindiseased tissue.
 27. The method of claim 12, wherein the microcapsulesrange in density from 1 to 5 g/cm³.
 28. The method of claim 12, whereinthe microcapsules range in density from 1.8 to 3 g/cm³.
 29. The methodof claim 12, wherein the microcapsules range in density from 1.8 to 2.2g/cm³.
 30. The method of claim 12, wherein the microcapsules have adensity of about 2 g/cm³.
 31. The method of claim 1, wherein themagnetic material is bound together using a copolymer of(R)-3-hydroxybutyric acid and (R)-3-hydroxyvaleric acid.
 32. The methodof claim 1, wherein the magnetic material is bound together using acopolymer of (R-3-hydroxybutyric acid and (R)-3-hydroxyvaleric acid,having a density range of 1.8-2.2 g/cm³ and a range in size from 20-50microns.
 33. The method of claim 3, wherein the magnetic material isdelivered to the diseased tissue by any one of the administrationmethods selected from the group consisting of intratumoral, peritumoral,or intravascular administrations.
 34. The method of claim 1, wherein themagnetic material is delivered to the diseased tissue by arterial orvenous blood supply.
 35. The method of claim 1, wherein the magneticmaterial is delivered to the diseased tissue in combination with atleast a vasoactive agent.
 36. A method for generating hysteresis heat ina tissue, comprising:delivering to the tissue magnetic material having amagnetic heating efficiency of at least about 4.5×10⁻⁸ J.m./A.g., whenthe magnetic field conditions are about 7.5×10⁷ A/s or less; andexposing the tissue, containing the delivered magnetic material, to alinear alternating magnetic field with a frequency of about 10 kHz orgreater and a field strength such that the product of field strength,frequency, and the radius of the exposed tissue is less than about7.5×10⁷ A/s to generate hysteresis heat in the tissue.
 37. The method ofclaim 36, wherein the tissue contains at least a cancerous growth orcontains one or more tumor.
 38. The method of claim 37, wherein saiddelivery and exposure are repeated until said cancerous growth or tumoris reduced in size.
 39. A method for the treatment of cancerous growthor tumors, comprising:delivering to a tissue containing a cancerousgrowth or tumor, a magnetic material having a magnetic heatingefficiency of at least about 4.5×10⁻⁸ J.m./A.g., when the magnetic fieldconditions are about 7.5×10⁷ A/s or less; and exposing the tissue,containing the delivered magnetic material, to a linear alternatingmagnetic field with a frequency of about 10 kHz or greater and a fieldstrength such that the product of field strength, frequency, and theradius of the exposed tissue is less than about 7.5×10e7 A/s to generatehysteresis heat in the tissue.
 40. A method for the heat treatment of atarget tissue in a patient, comprising the steps of:administering to thepatient's tissue magnetic material having a magnetic heating efficiencyof at least about 4.5×10⁻⁸ J.m./A.g., when the magnetic field conditionsare about 7.5×10⁷ A/s or less; and selectively exposing the tissue to alinear alternating magnetic field with a frequency of about 10 kHz orgreater and a field strength such that the product of field strength,frequency, and the radius of the exposed tissue is less than about7.5×10e7 A/s to generate hysteresis heat in the tissue.
 41. The methodof claim 40, wherein said administering comprises injectingmicrocapsules comprising the magnetic material.
 42. The method of claim40, wherein said microcapsules further comprise a cytotoxic agent. 43.The method of claim 40, further comprising the step of:exposing thetissue to ionizing radiation.
 44. The method of claim 43, wherein saidexposure is from an external radiation source.
 45. The method of claim41, wherein said microcapsules further comprise a radioactive compound.